The present invention relates generally to systems and methods for delivering of liquid chemicals, and more particularly, to systems and methods for delivery of liquid chemicals in precise amounts using logic devices and multi-reservoir load cell assemblies.
The present invention has many applications, but may be best explained by considering the problem of how to deliver photoresist to silicon wafers for exposure of the photoresist in the process of photolithography. To form the precise images required, the photoresist must be delivered in precise amounts on demand, be free of bubbles, and be of precise uniform thickness on the usable part of the wafer. The conventional systems have problems as discussed below.
As shown in FIG. 1, a representative conventional photoresist delivery system includes supply containers 100, 102, typically bottles, which supply photoresist to a single-reservoir 104 by line 117, which is connected to supply lines 106, 108 monitored by bubble sensors 110, 112 and controlled by valves V1 and V2. The bottom of the reservoir is connected to a photoresist output line 114 to a track tool (not shown) which dispenses photoresist on the wafer. The space above the photoresist in the reservoir 104 is connected to a gas line 118 which, based on position of a three-way valve V3, either supplies nitrogen gas to the reservoir 104 from a nitrogen manifold line 126, regulated by needle valve 1, or produces a vacuum in the reservoir 104. To sense the level of the photoresist in the reservoir 104, the system employs an array of capacitive sensors 122 arranged vertically on the walls of the reservoir 104. A two-way valve V4, located between the nitrogen gas manifold and the inlet of a vacuum ejector 124, supplies or cuts off flow of nitrogen to the vacuum ejector 124.
The photoresist delivery system must be "on-line" at all times so the track tool can dispense the photoresist as required. Many of the photoresist delivery systems attempt to use the reservoir to provide an on-line supply of photoresist to the track tool, but the photoresist delivery system must still refill the reservoir on a regular basis which is dependent on timely replacement of empty supply containers. Otherwise, the track tool will still fail to deliver the photoresist when demanded.
During dispense mode, when photoresist is withdrawn by the track tool from the reservoir 104, the valve V3 permits the nitrogen to flow from the nitrogen manifold to the reservoir 104 to produce a nitrogen blanket over the photoresist to reduce contamination and to prevent a vacuum from forming as the photoresist level drops in the reservoir. Once the photoresist in the reservoir 104 reaches a sufficiently low level the system controller (not shown) initiates refill mode, where a set of problems arise.
During refill mode, the valve V4 is activated so that nitrogen flows from the manifold line 126 to the vacuum ejector 124 which produces a low pressure line 170 thereby producing a low pressure space above the photoresist in the reservoir 104. The bubble sensors 110, 112 monitor for bubbles in the supply lines 106, 108, presumed to develop when the supply containers 100, 102, become empty. If, for example, the bubble sensor 110 detects a bubble, the controller turns off the valve V1 to supply container 100 and the valve V2 opens to supply container 102 to continue refilling the reservoir 104. However, bubbles in the supply line 106 may not mean supply container 100 is empty. Thus, not all of the photoresist in supply container 100 may be used before the system switches to the supply container 102 for photoresist. Thus, although the conventional system is intended to allow multiple supply containers to replenish the reservoir when needed, the system may indicate that a supply container is empty and needs to be replaced before necessary.
If the supply container 100 becomes empty and the operator fails to replace it and the system continues to operate until the supply container 102 also becomes empty, the reservoir 104 will reach a critical low level condition. If this continues, bubbles may be arise due to photoresist's high susceptibility to bubbles; if a bubble, however minute, enters the photoresist delivered to the wafer, an imperfect image may be formed in the photolithography process.
Further, if the pump of the track tool, connected downstream of the chemical output line 114, turns on when the reservoir is refilling, the pump will experience negative pressure from the vacuum in the single-reservoir pulling against the pump. Several things can happen if this persists: the lack of photoresist delivered to the track tool may send a false signal that the supply containers are empty, the pump can fail to deliver photoresist to its own internal chambers, lose its prime and ability to adequately dispense photoresist, and the pump can even overheat and burn out. The result of each scenario will be the track tool receives insufficient or even no photoresist, known as a "missed shot," which impacts the yield of the track tool.
The present invention addresses these problems as well as avoids waste of expensive photoresist, provides a friendly user interface depicting the amount of photoresist remaining in the supply containers, and reduces system capital and operating costs. If, for example, the amount of photoresist in the supply containers cannot be seen, the present invention permits the interface to be provided at a distance by conventional computer network capabilities and the electronics provided.